MAINTAINERS: Add Dave Hansen to the x86 maintainer team

Dave is already listed as x86/mm maintainer, has a profund knowledge
of the x86 architecture in general and a good taste in terms of kernel
programming in general.

Add him as a full x86 maintainer with all rights and duties.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Borislav Petkov <bp@suse.de>
Acked-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/87zgr3flq7.ffs@tglx